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Plop, Plop, Fizz fast: The Effect of Temperature on reaction time
Abstract Alka-Seltzer® tablets fizz furiously when dropped into water. The moment the tablet starts dissolving, a chemical reaction occurs that releases carbon dioxide gas. Can you make Alka-Seltzer fizz faster or slower by changing the temperature of the water? How big of a difference in the rate of a chemical reaction can temperature make? Objective To measure the effect of temperature on the rate of a chemical reaction. Materials and Equipment *Alka-Seltzer tablets (at least 12; if you plan to do additional variations to the project, you will want to get a larger box) *Thermometer with a range of at least 0°C to 60°C **A suitable thermometer is available from Amazon.com **A standard kitchen candy thermometer will also work fine *Clear drinking glass or jar (12 ounce mL or larger) *Graduated cylinder, 100 mL. A 100 mL graduated cylinder is available from Amazon.com. Alternatively, measuring cups may be used. *Masking tape *Something to stir with, such as a spoon or a chopstick *Hot and cold tap water *Ice *Stopwatch or a clock or watch with a second hand *A helper *Lab notebook *Pencil Introduction You may have seen a television commercial for Alka-Seltzer tablets, or heard one of their advertising slogans: "Plop, plop, fizz, fizz, oh what a relief it is!®" When you drop the tablets in water, they make a lot of bubbles, like an extra-fizzy soda, as shown in Figure 1 below. And like a soda, the bubbles are carbon dioxide gas (CO2). However, with Alka-Seltzer, the CO2 is produced by a chemical reaction that occurs when the tablets dissolve in water. Alka-Seltzer is a medical drug that works as a pain reliever and an antacid (antacids help neutralize stomach acidity, such as heartburn). The pain reliever used is aspirin and the antacid used is baking soda (sodium bicarbonate, NaHCO₃). To take the tablets, they should be fully dissolved in a glass of water. When sodium bicarbonate dissolves in water, it dissociates (splits apart) into sodium (Na+) and bicarbonate (HCO₃-) ions. (An ion is a molecule that has a charge, either positive or negative.) The bicarbonate reacts with hydrogen ions (H+) from citric acid (another ingredient in the tablets) to form carbon dioxide gas and water. In other words, carbon dioxide gas is a product of this reaction. The reaction is described by Equation 1 below: Equation 1. 3HCO3−+3H+→3H2O+3CO2So how is temperature related to this bicarbonate reaction? In order for the reaction shown above to occur, the bicarbonate ions have to come into contact with the hydrogen ions. Molecules in a solution are in constant motion, and are constantly colliding with one another. The hydrogen and bicarbonate ions must collide at the right angle and with enough energy for the reaction to occur. The temperature of a solution is a measure of the average motion (kinetic energy) of the molecules in the solution. The higher the temperature, the faster the molecules are moving. What effect do you think temperature will have on the speed, or rate, of the bicarbonate reaction? In this chemistry science project you will find out for yourself by plopping Alka-Seltzer tablets into water at different temperatures, and timing how long it takes for the chemical reaction to go to completion. Terms and Concepts *Chemical reaction *Alka-Seltzer *Baking soda, or sodium bicarbonate *Molecule *Products *Temperature *Bicarbonate reaction *Reaction rate Questions *What is the bicarbonate reaction? What are its products? *Keeping in mind that an increase in temperature reflects an increase in the average molecular motion, how do you think increasing temperature will affect the reaction rate? *What temperature change do you think would be required to increase, or decrease, the reaction time by a factor of 2? *What other factors beside temperature can affect how well a chemical reaction takes place? Experimental Procedure In this science project, you will be measuring the time it takes for one Alka-Seltzer tablet to react completely in water. You will investigate how the reaction time changes with water temperature. #Do your background research and make sure that you are familiar with the terms and concepts in the Background. #In your lab notebook, make a data table like Table 1 below. You will record your results in this data table. Table 1. In your lab notebook, make a data table like this one. You will record your results in it.#Prepare a drinking glass so that it is marked at the 250 mL point. You will use the same glass for multiple trials, so it is convenient to mark the desired water level. ##Add 250 mL (approximately 1 cup) of water to the drinking glass. ##Use a piece of masking tape on the outside of the glass to mark the water level, placing the tape with its top edge even with the water level in the glass. ##''Note:'' You do not want to fill the glass completely full because the bicarbonate reaction produces bubbles that could splash out. #You will fill the drinking glass with the same volume of water at three different temperatures: hot tap water, cold tap water, and ice water. ##For the hot and cold tap water, run the water until the temperature stabilizes. Fill the glass with water to the level of the masking tape. Be careful when handling the hot water. ##For ice water, fill the glass about half full with ice cubes, then add cold tap water to a bit above the level of the masking tape. Stir for a minute or two so that the temperature equilibrates. Once temperature has equilibrated, remove the ice cubes from the water's surface using a spoon or other utensil immediately before adding the Alka-Seltzer tablet. (Pour out any extra water so that the water is up to the level of the masking tape.) #Prepare the drinking glass with one of the three temperatures as described in step 4. Then measure the reaction time for that temperature. ##After filling the glass to the level of the masking tape, measure the temperature of the water (in Celsius C), and record it in the data table in your lab notebook. ##Remove the thermometer. (It is not a good idea to use the thermometer as a stirring rod. It might break.) ##Have your helper get ready with the stop watch, while you get ready with an Alka-Seltzer tablet. Have your helper count one–two–three. On three, the helper starts the stop watch and you drop the tablet into the water. ##You will immediately see bubbles of CO₂ streaming out from the tablet. ##Stir the water gently and steadily. Use the same stirring method and speed for all of your experimental trials. The tablet will gradually disintegrate. Watch for all of the solid white material from the tablet to disappear. ##When the solid material has completely disappeared, and the bubbles have stopped forming, say "Stop!" to have your helper stop the stopwatch. ##Record the reaction time (in seconds s) in the data table in your lab notebook. ##''Tip:'' Be careful when opening the packets and handling the Alka-Seltzer tablets. The tablets are thin and brittle, so they break easily. If some of the tablets are whole, and some are broken into many pieces, the separate trials will not be a fair test. You should only use whole tablets. #Repeat step 5 three more times with the same temperature. ##Repeating an experiment helps ensure that your results are accurate and reproducible. #Repeat steps 5 to 6 for each of the other temperatures. ##When you are done, you should have done a total of four trials for each of the three temperatures. #Calculate the average reaction time for each of the three water temperatures. Record your results in the data table in your lab notebook. #Make a graph of the average reaction time, in seconds (on the y-axis), vs. water temperature, in degrees Celsius (on the x-axis). #How does reaction time change with temperature? Can you explain why this is? ##''Hint:'' If you are having trouble explaining your results, try re-reading the Introduction in the Background.